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Fundamentals of Light

Prism breaking white light into a rainbow

White light is composed of an infinite number of individual colors. Your eye blends the colors and your brain “sees” white light. A prism, such as the one shown above and on Pink Floyd's album cover, is able to resolve the light back into individual colors.

Fundamentals of Light

Electromagnetic Radiation Spectrum

We only see objects that have light waves that travel from them to our eyes. Objects either generate light or reflect it. Light is generated by incandescent and fluorescent lamps, light emitting diodes, flames, very hot objects, and even some animals.  Our major source of light is the sun.  Light is a small fraction of the energy we call electromagnetic radiation.

Electromagnetic radiation is a form of energy that consists of oscillating electric and magnetic fields traveling at the speed of light. Electromagnetic waves carry this energy from one place to another and are similar to waves in a rope. Unlike the wave in a rope, however, electromagnetic waves do not require a medium to travel; the most prevalent electromagnetic waves we experience travel through the vacuum of outer space from the sun

The energy of an electromagnetic wave travels in a straight line along the path of the wave. The moving light wave has associated with it an oscillating electric field and an oscillating magnetic field. Scientists often represent the electromagnetic wave with the image below. 

Diagram of an electromagnetic wave

The black line represents the straight path of the light itself. Along this path, there exists an electric field that will reach a maximum positive charge, slowly collapse to zero charge, and then expand to a maximum negative charge. Similarly, there is an changing magnetic field that oscillates from maximum north pole field to maximum south pole field. Along the path of the electromagnetic wave, these changing fields repeat, oscillating over and over again. However, the oscillating electric and magnetic fields demonstrate a weaving pattern that is not the way light travels. For an electromagnetic wave, the crests and troughs represent the oscillating fields, not the path of the light.

Although light waves look different from waves in a rope, we still characterize light waves by their wavelength, frequency, and velocity. We can measure along the path of the wave the distance the wave travels between one crest and the succeeding crest, which is the wavelength of the electromagnetic radiation. Like transverse waves through a medium, the frequency of electromagnetic waves is the number of full cycles of waves that pass a certain point in a set unit of time. The velocity for all electromagnetic waves traveling through a vacuum is the same:  3.00 \times 10^8 \ m/s , which is symbolized by the lower case c . The relationship, then, for the velocity, wavelength, and frequency of electromagnetic waves is: c = \lambda f .

The entire spectrum of electromagnetic waves includes very low energy electric waves up to very high energy gamma rays. As you probably know, ultraviolet rays, X-rays, and radio waves are also waves on the electromagnetic spectrum. The full electromagnetic spectrum is shown below. You can see that visible light is a very small fraction of the electromagnetic spectrum.

The electromagnetic spectrum

Color

The colors that we see are blends of some number of individual frequencies of visible light.  The surface of materials have chemical structures that cause particular frequencies of light to be either absorbed or reflected. Our eyes pick up the frequencies that are reflected (or generated) by the object and the frequencies are blended so that we see one color. If the surface of an object absorbs all frequencies of light, we see black. If all frequencies are emitted or reflected, we see white. If you had a book whose cover reflected only blue light and you placed this book in the presence of white light from an incandescent light bulb, all the colors except blue would be absorbed and the blue would be reflected, allowing you to see a blue book. If you placed the same book in the presence of only red light, no light would be reflected from the book and you would see a black book.

Discussions of colors can sometimes become confusing because of the difference between light emitters and light absorbers. Light absorbers, such as dyes and pigments, absorb frequencies from those that you see, while light emitters add frequencies to what you see.  If I were to shine red light, green light, and blue light into your eye all at the same time, you would likely see some variation of white. If I were to mix red paint, green paint, and blue paint together, you would likely see black or some very dark color.

If an object emits light waves, we say the object is luminous (generates light). If an object merely reflects light that falls on it, we say the object is illuminated (reflects light). Many objects allow light rays to pass through them without disrupting the organization of the rays. When this happens, you can clearly identify the objects from which the light came.   Objects that allow light to pass through undisrupted are said to be transparent . Air, glass, some plastics, and some other materials are examples of transparent objects. There are also objects that allow light to pass through them but they disrupt the rays so that the objects from which the light came cannot be identified. Objects that allow light to pass through but disrupt images are said to be translucent . Lampshades, frosted light bulbs, and fogged glass are examples of translucent materials. Many other objects do not allow any light to pass through. Such objects are said to be opaque .

Polarization

Remember the illustration of electromagnetic radiation given above? These electric and magnetic waves may oscillate in any direction. That is, the oscillating fields may be oscillating vertically, horizontally, and in every other direction. Light typically consists of oscillating fields in all directions, but always travels in a straight line.

Filters can be constructed such that only light with the fields oscillating in a certain direction can pass through the filter. This is much like having a large number of ropes passing through the slots in a picket fence. The ropes have transverse waves oscillating in all possible directions moving toward the fence. When the waves encounter the fence, only the oscillations that are vertical and fit through the slots in the fence will be allowed to pass through. Light that has all its field oscillations in the same direction is said to be polarized .

Summary

  • We see objects because they either generate or reflect light, which then enters our eyes.
  • Visible light is a small fraction of the energy we call electromagnetic radiation.
  • Electromagnetic radiation is a form of energy that consists of oscillating electric and magnetic fields traveling at the speed of light.
  • The energy of an electromagnetic wave travels in a straight line along the path of the wave.
  • In a vacuum, every electromagnetic wave has a velocity of 3.00 \times 10^8 \ m/s , which is symbolized by the lower case c .
  • The relationship of velocity, wavelength, and frequency of electromagnetic waves is: c = \lambda f .
  • Light that has all its field oscillations in the same direction is said to be polarized.

Practice

Questions

Use the video on the visible light spectrum to answer the questions that follow.

http://video.about.com/physics/The-Visible-Light-Spectrum.htm

  1. What wavelengths are included in the visible spectrum?
  2. All colors of light mixed together produces _____________. 
  3. What is necessary for a rainbow to form?

Review

Questions

  1. Sound doesn’t travel through a vacuum because there are no molecules to carry it.  How do we know that light does travel through a vacuum?
  2. What is the range of wavelength of electromagnetic radiation the human eye can detect?  
  3. What was changed in the equation v = \lambda f  in this concept?  
  4. What color of visible light has the shortest wavelength?  
  5. How are we able to see objects that do not generate light?  
  6. Of what colors does white light consist?  
  7. Why can’t sound waves be polarized?  
  8. What happens to the wavelength of light as the frequency increases?  
  9. The sky appears darker when viewed through a polarizing filter.  Why?  
  10. What color will a yellow banana appear when illuminated by
    1. white light?
    2. yellow light?
    3. blue light?

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